Batboard

On a Motorola Apco 25 System with 3 voted receiver site, Can the remote receivers be tied to the compartor via RF links or does it have to be via modems

I am assuming you are talking about a conventional system yes?

The links between the remote receiver sites and the ASTROTAC comparitor at your transmit or dispatch site are V.24 Serial Data Links. You aren't just going to be able to put up a UHF link or something. There's a little more to it than that. You can acomplish this via a T1/Channel bank, or ASTRO modems, over twisted pair thru the telco. You could probably also get it working across an unlicensed microwave hop such as canopy, RAD, etc, if the unit has 4 wire functionality, and your receiever sites are within the range limiatations of one of those products.

hmm, No this is a new system that has been proposed to be installed as p25 digital.

With 1 main transmitter site and 2 voted receiver sites tied back to a astrotac 3000 comparator at the main transmitter.

On the proposal they mention RF links for the Link between the receivers and the comparator and then they list the modems, but no mention of leased lines or microwave link, from the proposal it appears they are going to link the remote rx's site via xtl1500's

Which I am under the impression can't be accompolished in Digital P25 Mode

Correct??

Yeah, it sure seems whatever information you have is only telling part of the story. Ask for a system diagram.

The ASTRO modems are dependent on a telco carrier grade circuit, whether that is provided via microwave(RF), fiber, copper, etc. You are correct, those mobile radios cant' provide that type of link.

Okay thanks, I have requested a system layout and scheeduled a Conference call between the Dealer and Motorola, I think I have finally convinced my superiors last night that what was propsed wouldn't work

Could the Link's be estabilished VIA a Canopy SYSTEM

You can use RF links for this, but they'll need to be transmitting 24/7, and be full duplex. You'll essentially be putting the audio to/from the astro modems over the RF link instead of a two-pair phone line. So, as long as your link radios are rated for 100% duty cycle, you have a proper frequency pair and duplexer/filtering for each link radio, and the link radios allow you to get flat audio in and out, it should work.

For the AstroTAC comparator to really 'vote', meaning that it would be able to switch recievers in the middle of a call, all of the signals from the voter receivers must arrive at the AstroTAC within about 100 milliseconds of each other. This will be tough to do if you're converting 9600 bps V.24 data or modem audio to ethernet packets and back again on some (but not all) of the links.

If the arrival times from some of the voter receivers exceed 100 ms relative to the other ones, the comparator will still work, but it will simply vote on the "first in" signal, and not be able to change receivers until the transmission ends or drops out completely.

If you were to make all the links via an IP connection, Canopy, or whatever, you may still get true voting as long as the latency of each link is within 100 ms of the others. So, if you have a reciever co-located with the comparator, you'll need to delay it's signal to the comparator by the same amount as the rest of the links, within 100 ms.

To convert V.24 data to ethernet, I've seen the Juniper Networks CTP-1002 work with the Quantar:
http://www.juniper.net/us/en/products-s ... tp-series/

To convert astro modem audio to ethernet packets, I've seen the RAD IPmux-1E work:
http://www.radusa.com/Article/0,6583,31792,00.html

Of course if the latency of the links is too long, the system will just perform poorly in general, even if it is really voted. Worst case, when a user unkeys, they'll hear the end of their own transmission! Very delayed repeater access times greatly increases the chances of users doubling with each other.

My experience is with conventional P25 digital systems. This might apply to P25 digital trunked systems, as well.